Business chiefs, including Dale Farm's David Dobbin, have rallied in support of the UK staying in the EU as David Cameron urged fellow MPs against the "leap in the dark" of a Brexit.

Leading the charge to stay in the EU, Mr Dobbin, chief executive of United Dairy Farmers, said leaving "one of the world's largest trading blocks and go it alone would simply be too great a risk to take".

"While we have concerns relating to EU regulations which add to our costs, we have even greater concerns on the risks which would be incurred by leaving the EU," he said.

And on single farm payments, which farmers here receive from the EU, he said those in the industry would "be worse off" if voters choose a Brexit when they go the polls on June 23.

"There is too much uncertainty on trade agreements... we would risk considerable political and economic uncertainty at a time when the world needs stability."

Around 81% of businesses here support staying in a reformed EU, according to the Northern Ireland Chamber of Commerce and Industry.

And Martin Hamilton of Mash Direct said pulling out of the EU would be a “nightmare” for business.

He said, given the company’s level of exports, Mash Direct would have to set up a separate EU hub if a Brexit went ahead.

“It beggars belief that some of our politicians here, and across the water, who are supposed to be representing business, are campaigning to leave,” he said.

Nigel Smyth, CBI director in Northern Ireland, said remaining in the EU would help businesses in a number of ways.

That includes the benefit in accessing one large, single market, opening up international markets and enjoying “common rules” across business.

And there are “too many unanswered questions about what leaving the EU would entail and the impact it would have”, according to Mark Hutchinson of Kilrea-based aerospace parts maker Hutchinson Group.

The debate on a Brexit was electrified at the weekend when Conservative London Mayor Boris Johnson announced he was in support of the UK leaving the EU.

Prime Minister David Cameron yesterday put his reasons for staying to the House of Commons.

Brendan McGurgan, managing director of machine maker CDE Global, said a Brexit “would create uncertainty in the short to medium term which would undoubtedly have a negative impact on our European business”.

Brian McGrath, head of Londonderry Port & Harbour Commissioners, said the “prospect of Brexit is a massive issue for the north west and Northern Ireland in general”.

And Dorit McCann, partner at law firm Carson McDowell, said with “Northern Ireland sharing the only land border with another EU member state, the concerns for us are particularly marked”.

Richard Ennis, director of business and corporate banking at First Trust, said the “particular nuances” of leaving the EU “need to be further explored and debated”.

A report last year said that Northern Ireland could be hit harder than almost anywhere else in the UK if it exited the EU.

Northern Ireland exports more products to the EU — around 57% — than any other region in the United Kingdom.

But while most businesses want to stay in, Irwin Armstrong, chief executive of Ciga Healthcare, says he finds it easier to do business with firms in the Middle East and South America, rather than those in the EU.

Some business owners are yet to be convinced, including Conor Devine of Belfast financial firm GDP Partnership.

“This is a complicated and complex matter and to date no one has presented any form of business case,” he said.